1. Field of the Invention
The present invention relates to a method for manufacturing PTC elements suited for surface mounting.
2. Description of the Related Art
PTC (Positive Temperature Coefficient) elements are known as protective elements for controlling the current which flows through circuits to be protected since their resistance value increases as they give off heat in overcurrent conditions.
PTC elements essentially possess a structure in which the upper and lower surfaces of a flat-plate-shaped PTC material are sandwiched by electrodes; however, in order for these elements to be used in surface mounting, the terminals of the upper and lower electrode need to be together on one surface.
A conventional method for manufacturing such surface mount-type PTC elements involves, as depicted in FIGS. 4A to 4E, sandwiching a PTC material 1 between an upper electrode foil 2a and a lower electrode foil 2b (FIG. 4A), forming through-holes 3, establishing connection between the upper electrode foil 2a and the lower electrode foil 2b by filling the through-holes 3 with a plating layer 4 (FIG. 4B), patterning the electrode foils 2a, 2b of the upper and lower surfaces as well as the plating layer 4 (FIG. 4C), applying an insulating material 5 to the assembly except those portions which are to serve as the electrode terminals on the plating layer 4 (FIG. 4D), then cutting the assembly along cutting lines L which pass through the through-holes 3 (FIG. 4E).
This allows an electrode terminal 6a of the upper electrode foil 2a and an electrode terminal 6b of the lower electrode foil 2b to be formed on the bottom surface of the PTC material 1, an electrode terminal 6axe2x80x2 of the upper electrode foil 2a and an electrode terminal 6bxe2x80x2 of the lower electrode foil 2b to be formed on the upper surface of the PTC material 1, and thereby a PTC element 10X to be obtained, on which surface mounting can be performed at the upper or lower surfaces of the PTC material 1 (FIG. 4E).
However, the steps for manufacturing the PTC element 10X as shown in FIGS. 4A to 4E are complex, and the manufacture costs high. Moreover, the PTC element 10X is not moulded, which results in poor moisture resistance as well as the risk of ignition when the PTC element gives off more heat than normal.
In response to these defects, proposed is the injection mould method of manufacture, as shown in FIG. 5, in which a surface mount-type PTC element 10Y is manufactured by sandwiching the upper and lower surfaces of the PTC material 1 between the electrode foils 2a, 2b, forming leads 7, 8 to allow the electrode terminals 6a, 6b of the electrode foils to be accessible on one surface of the PTC material 1, and moulding the whole assembly with a moulding material 9 so as to leave only the electrode terminals 6a, 6b exposed.
However, even in the procedure for manufacturing of the PTC element 10Y, once the PTC material 1 has been sandwiched between the electrode foils 2a, 2b, the leads 7, 8 need to be laminated thereon, which necessitates a costly increase in the number of manufacturing steps and components. A further problem is that the thickness of the element will increase. For example, if the thickness of the PTC material t1 is 0.4 mm, the thickness t2 of each electrode foil 2a, 2b 0.05 mm, the thickness t3 of each lead 7, 8 0.2 mm and the thicknesses t4a, t4b of the mould material 9 on the upper and lower surfaces 0.3 and 0.6 mm respectively, the thickness of the whole PTC element assembly t0 will be 1.8 mm.
In an attempt to resolve the aforedescribed problems with the prior art, it is an object of the present invention to reduce the thickness of surface mount-type PTC elements and enable them to be manufactured at lower cost.
The present inventors perfected the present invention as a result of discovering that surface mount-type PTC elements of reduced thickness can be obtained using a streamlined manufacturing process that is economically advantageous, by forming an electrode layer using an electrode foil or the like so as to wrap a flat-plate-shaped PTC material, partially removing the electrode layer to form upper and lower electrodes, then having the electrode terminals thereof be formed on either the upper or lower surface of the PTC material.
In other words, the present invention provides a method for manufacturing a PTC element, comprising the steps of:
forming an electrode layer on a surface of a flat-plate-shaped PTC material, such that it extends over the upper and lower surfaces and at least one side surface thereof; and
removing a portion of the electrode layer to segment it into a region where it is present on either of the upper or lower surfaces of the PTC material and a region where it extends over the upper and lower surfaces as well as a side surface, to form an upper surface electrode and a lower surface electrode of which the terminals are positioned on one of the surfaces of the PTC material.